In accordance with recent development of portable and wireless electronic equipment, there is an increasing demand for a compact and lightweight secondary battery of high energy density that serves as a power source for driving the electronic equipment. To meet such demand, non-aqueous electrolyte secondary batteries which are compact, lightweight and capable of quick charging and have high energy density have been developed. Among those, a prevailing one is a lithium ion rechargeable battery.
A typical non-aqueous electrolyte secondary battery includes a spiral electrode group formed by winding a positive electrode, a negative electrode and a separator interposed therebetween, a non-aqueous electrolyte and a battery case for housing the electrode group and the non-aqueous electrolyte. The positive electrode includes a current collector made of aluminum and a positive electrode material mixture layer formed on the aluminum current collector. As a positive electrode active material, a lithium-containing transition metal compound such as LiCoO2 is used. On the other hand, the negative electrode includes a current collector made of copper and a negative electrode material mixture layer formed on the copper current collector. As a negative electrode active material, a carbon material is used, for example. This battery utilizes intercalation-deintercalation of lithium ions into and from the electrodes and is designed so that the positive and negative electrodes are opposed in as large area as possible to allow high rate charge/discharge.
However, as the electronic equipment is provided with more functions and becomes more power-consuming, a battery of much higher capacity and energy density is keenly demanded. Therefore, the separator or the current collector is made thinner to reduce a distance between the positive and negative electrodes and a space within the battery case which does not contribute to the capacity is made small.
In a battery comprising the spiral electrode group and the non-aqueous electrolyte contained in the battery case, expansion of the negative electrode occurs during charge and hence the electrode group is deformed. As a result, the electrodes are locally pressured, producing a region where uniform intercalation-deintercalation of lithium ions cannot be achieved. In such a region, the positive electrode is apt to increase its potential and hence the transition metal in the positive electrode active material is leached out to deposit on the negative electrode during charge/discharge or storage at high temperatures in a charged state. The deposited metal penetrates the separator before long and brings about an internal short circuit. This causes abnormal voltage reduction and leads to a decrease in reliability of the battery.
So far, for example, Japanese Laid-Open Patent Publications Nos. Hei5-182691 and Hei11-273739 have proposed a technique of arranging an ionic insulator between an end of the positive electrode material mixture layer and the negative electrode material mixture layer opposing thereto. This technique is intended to control the charge reaction itself involving the intercalation-deintercalation of lithium ions by the ionic insulator to inhibit the positive electrode potential from locally increasing and prevent the internal short circuit. However, according to this technique, an internal short circuit as described below cannot be prevented because the ionic insulator is arranged between the positive and negative electrode material mixture layers. Further, since the ionic insulator directly covers the positive or negative electrode material mixture layer, the electrode reaction is inhibited and the capacity decreases.
FIG. 3 shows a partial sectional view of a spiral electrode group according to a related art.
This figure shows the outermost turn of the electrode group abutting on a battery case 14 and its vicinity. Referring to FIG. 3, a positive electrode current collector 11a of a positive electrode 11 is situated at the outermost turn of the spiral electrode group, both sides of which are exposed. At the second outermost turn, a positive electrode material mixture layer 11b is formed only on an inner circumference side of the positive electrode current collector 11a. A positive electrode lead 11c is welded to the exposed positive electrode current collector 11a at the outermost turn of the electrode group. Further, a lengthwise end 19 (an end in the lengthwise direction) of a separator 13 lies far inside from the positive electrode lead 11c along the turn of the electrode group to allow a certain space therebetween. At the inside of the positive electrode 11 along a radius direction of the electrode group, a negative electrode 12 is arranged with the interposition of the separator 13. As to the negative electrode 12, negative electrode material mixture layers 12b are formed on both sides of a negative electrode current collector 12a even in the vicinity of the outermost turn of the electrode group.
Short circuit regions 15 are generated within the separator between the positive electrode material mixture layer 11b and the negative electrode material mixture layer 12b. The short circuit regions 15 are opposed to step portions having difference in level formed in the electrode group by the lengthwise end 19 of the separator 13 and the periphery of the positive electrode lead 11c, respectively, with the intervention of the positive electrode material mixture layer 11b and the positive electrode current collector 11a. In the electrode group thus configured, regions opposing to the step portions having difference in level may receive local pressure if an internal pressure of the electrode group is increased due to expansion of the negative electrode during charge or the electrode group is pressured by the inner wall of the battery case. In the locally pressured regions, distance between the positive electrode 11 and the negative electrode 12 is reduced and hence the electrode reaction is apt to concentrate thereon. Thereby, the short circuit regions 15 are generated in the separator lying in the reduced space between the electrodes.
FIG. 4 is a front view observed in the direction of an arrow X in FIG. 3 showing a region A including the short circuit regions 15 in the negative electrode of the electrode group shown in FIG. 3. The width of the negative electrode is a size larger than that of the positive electrode. On boundaries 16 of an area of the positive electrode opposing to the negative electrode, the positive electrode potential is apt to increase locally, thereby the positive electrode active material tends to be leached out. The same is applied to a boundary 17 (lengthwise end) of the separator (not shown) opposing to the region A, as well as a boundary 18 of the positive electrode lead opposing to the region A. The leached positive electrode active material deposits particularly on the negative electrode in a region where the distance between the positive and negative electrodes is small. Therefore, the short circuit regions 15 are most apt to be generated at the points of intersection of the boundaries 16 with the boundary 17 or the boundary 18.
Apparent from the above description, with respect to a non-aqueous electrolyte secondary battery of high energy density, it is necessary to cause the electrode reaction as uniformly as possible and prevent the internal short circuit caused by the leach of the positive electrode active material. Further, it is also necessary to avoid voltage failure in a battery in an initial state and abnormal voltage reduction in a charged battery during storage at high temperatures. The present invention has been achieved in view of these problems.